1. Field of the Invention
The invention relates to a process for preparing solid polyvinyl ester resins in the presence of chain transfer agents.
2. Background Art
For many applications, the rheological properties of polymers are of critical importance. The theological properties for a given polymer are primarily determined by the molecular weight and hence the degree of polymerization. In the case of polymers which are prepared by free-radical polymerization, the molecular weight can be regulated in differing ways, for example using what are termed chain transfer agents, i.e. substances which may be added to regulate the degree of polymerization of the reaction. Very high transfer constants are exhibited by halogenated compounds and mercaptans (Ct=10−1 to 101). In the case of such pronounced effects, the consumption of the chain transfer agent is very high, so that the chain transfer agent must be metered into the reaction mixture to keep its concentration constant.
DE-A 2830324 discloses a process for preparing polyvinyl acetate, which is used in particular as base polymer for chewing gum, in the presence of acetaldehyde as chain transfer agent. It is a disadvantage of this process that the polyvinyl acetate thus prepared is unstable under thermal stress and when in contact with nucleophilic substances. Thus, in the case of storage for several hours at 120° C., in addition to an intense yellow discoloration, a pungent odor of acetic acid can be detected. As a result of the contact with bases and with other nucleophiles, for example the sweetener aspartame, a yellowish-brown discoloration of the product is observed. The above-described effects are due to the use of acetaldehyde as a chain transfer agent, as a result of which, methyl ketone end groups are introduced into the polymer. The acid CH2 group in such end groups can be converted by bases into the corresponding anion. The system then stabilizes with elimination of an acetate group and the formation of an α,β-unsaturated ketone. The resultant vinylogous carbonyl compound again has an acidic CH2 group, so that the entire process can repeat. It is possible by UV spectroscopy, to detect a conjugated double bond system having approximately 5 to 6 double bonds which forms in a very short time. At elevated temperature, the traces of acetic acid and water present in the polymer are already sufficiently nucleophilic to lead to the formation of conjugated double bond systems.
Japanese applications JP-A 01026602 and JP-A 57105410 disclose polymerizing low-molecular weight polyvinyl acetate in the presence of mercaptans. Disadvantages here are the remaining odor of these polymers, their strong tendency to produce discolored, yellow products, and the technical difficulty of constant metering of very small amounts of mercaptan over relatively long time periods, due to the high transfer constants of these compounds.
To prepare polyvinyl acetate solid resin for applications in the chewing gum field, however, regulating substances must be used to obtain desirably low molecular weights, since the molecular weight determines the chewing properties (rheological properties). Thus, it would be desirable to provide a process for preparing solid polyvinyl ester resins by which low-molecular-weight polymers are accessible, without the abovementioned undesirable side effects such as thermal and chemical instability.